The present disclosure generally relates to friction stir welding and, more specifically, to processes of friction stir welding that include providing an additional solid-state source during the friction stir welding.
Friction stir welding is a solid-state joining process that that provides material deformation similar to that of extrusion forging. Since the friction stir welding process occurs at a temperature below the melting point of the work piece material, solidification-related defects that are common in fusion type welds, for example, are substantially eliminated, and friction stir welding generally provides relatively higher joint strengths. Additionally, the lower heat input used in the friction stir welding process as opposed to other welding processes results in less residual stress, less distortion, and less impact to the final metallurgical properties of the work piece. Many joint designs are possible using friction stir welding including, but not limited to corner and T-joints.
As shown in prior art FIGS. 1 and 2, a typical friction weld stirring apparatus 10 uses a cylindrical, shouldered tool 12 with an optional profiled pin 14 that is rotated and slowly plunged into a joint line 16 between two pieces of sheet or plate material 18, 20, which are butted together and clamped by a mechanical clamp 22. A typical mechanical clamp includes a top plate 26 and a bottom plate 28 and threaded bolts 24 for locking the butted work pieces 18, 20 therebetween during operation. Frictional heat is generated between the rotary tool 10 and the immediate region about the joint line 16 defined by the butted work pieces 18, 20, which cause region about the joint line 16 to soften and plastically flow without reaching the melting point. As the rotary tool traverses the joint line, the plasticized material is transferred from the leading edge of the tool to the trailing edge of the tool shoulder and pin, leaving a solid phase bond between the two work pieces. To terminate the weld, the linear traversal along the weld line is terminated and the rotary tool is retracted from the workpiece surface leaving behind an exit keyhole. Providing a run-out region that can subsequently be removed can eliminate the exit keyhole, but this is not always feasible nor cost effective. Alternative processes to eliminate the exit keyhole effect have been developed such as the use of a variable penetration tool that can be slowly retracted while traversing the weld line. This can result in a smooth transition from a non-welded region to a fully welded region back to a non-welded region. Other known techniques to eliminate the exit keyhole effect include modification of the pin profile. Current methods are not entirely effective at eliminating the exit keyhole effect.
Another one of the ongoing problems with current friction stir welding processes is that the welding process has a tendency to produce flash material and also results in a general thinning of material along the weld line. The reduction in cross sectional thickness about the weld line reduces the serviceable load of the joint. This strength reduction must be mitigated by a thickness compensated design and this penalty can be significant in many situations especially for plate joining applications where additional thickness penalties must be made across the entire width of the plate. Moreover, the production of flash material presents safety concerns for handling personnel and requires further post-processing.
Accordingly, it would be desirable for friction stir weld processes that compensated for the production of flash and general thinning of material along the weld line as well as provided a relatively smooth transition from the non-weld regions to the welded region.